We are requesting funds for the purchase of a multi-color, high-speed, digital flow cytometric analyzer: a BD LSRII, custom outfitted with 407, 488, 532 and 633 lasers, along with a high throughput autoloader. This instrument will be installed in the Yale FACS Core, in publicly accessible dedicated space. The FACS Core currently services over 110 PIs and 486 individual users (128 of whom are already trained on the LSRII). Twenty-seven PIs were identified as major users of the new instrument and their combined planned use alone will account for most of the available hours on both our existing and the new machine. These PIs have over $19M in NIH-funded direct costs from at least 7 different institutes. While naturally a major research focus is in the broad area of immunology and allied immune diseases, disciplines such as stem cell biology, hematopoiesis, infectious disease, and vascular biology are well-represented. There are two main justifications for our request. First, our existing LSRII is already at capacity and users are experiencing problems booking time. Usage is growing at 50-60% per year. In addition, three major initiatives will substantially increase demand. Second, the LSRII will provide some unique capabilities, including the ability to detect RFP from the 532 laser (requested by 15 major users) and to do automated sample loading (requested by 8 major users, but among them those who use the largest number of hours). The new machine will be well cared for by the staff of the FACS Core, who are already trained and experienced in its use. We have a dedicated technician to oversee the machine and to train users, and our own training program. We will start and certify the machine each morning and troubleshoot any problems, as we will do at any time during working hours. The Core also provides consultation on experimental design and interpretation, particularly useful color combinations for the LSRII. There is a developed and functioning administrative framework for the machine to handle billing, service contract renewals, and the like. Overall, the capacity of the LSRII will be fully used and users are likely to properly take advantage of its capabilities. As such, it will have a remarkable impact on the productivity of an outstanding and well-funded group of investigators. This instrument will enable the research of 27 NIH-funded PIs who require sophisticated FACS analysis and specific features of the LSRII, which otherwise may not be available due to limited capacity or capability of our existing machine [unreadable] [unreadable] [unreadable]